There’s some good news related to an Ebola crisis that has offered very little up until now. The World Health Organization now predicts there are adequate supplies of an experimental Ebola vaccine to control the outbreak in the Democratic Republic of the Congo.

“I believe we will have enough vaccine to stop this outbreak unless something very dramatic changes,” Dr. Peter Salama, WHO’s deputy director-general of emergency preparedness and response, told STAT.

And while an ongoing clinical trial of the vaccine may not be complete for some time, preliminary analyses suggest it is working.

Salama said the evidence the WHO has been gathering in North Kivu—where nearly 64,000 doses have been administered—point to the vaccine being “highly, highly efficacious.” That conclusion, he said, is based on how the vaccine has performed in the people who have received it, on three modeling studies, and on an earlier study of the vaccine conducted in West Africa.

“So a lot of pieces of evidence that point in absolutely the same direction,” Salama said.

The vaccine is known by a provisional name, V920, and is made by Merck. It was first shown to be protective in people in a landmark clinical trial in Guinea toward the end of the massive West African Ebola crisis of 2014-2016.

That study tested the vaccine in an approach that is being used in the current outbreak, a ring-vaccination strategy. People who have been in contact with a case—and then all of their contacts—are offered vaccine. People on the front lines of the response—health workers, people who conduct safe burials—are also offered the chance to be vaccinated.

Salama said the vaccine efficacy rate in the current outbreak is well above 90 percent. He acknowledged there have been a few infections in people who were vaccinated, but said the number—which he declined to specify—is low. “There are very few breakthrough cases,” he said.

Salama credits the vaccine for the fact that transmission at Beni—a major hot spot for several months—appears to be coming under control, even though the outbreak response teams have had a difficult time there tracing the chains of transmission and recurring violence has hampered their efforts.

This outbreak, which is the second largest on record, marks only the second time a vaccine has been used essentially from the start of an Ebola epidemic. The first was in a smaller outbreak in western DRC in the spring.

As of Monday, there have been 689 confirmed and probable infections in this outbreak and 422 deaths.

Merck began filing documents for its licensure bid with the Food and Drug Administration in late 2018 and expects to apply to the European Medicines Agency for licensure in 2019, said Beth-Ann Coller, the company’s team leader for the vaccine project.

In an agreement forged with the WHO and with Gavi, the Vaccine Alliance, after the West African outbreak, Merck committed to maintaining a stockpile of 300,000 doses of the vaccine at all times while it was working to get the product licensed.

Merck has donated the vaccine to the WHO and the Congolese Ministry of Health.

Until this outbreak, the size of the stockpile seemed more than adequate; historically most Ebola epidemics have been controlled after fewer than 100 cases. But as the dangerous and challenging outbreak in DRC’s North Kivu province has dragged on, and the number of vaccine doses used has climbed into the tens of thousands, concerns have arisen that the company might run out of supply.

The WHO has already indicated the size of the Ebola vaccine stockpile must be revisited.

It takes a year to make a batch of the vaccine, from ordering the ingredients to capping filled multidose vials, Coller explained.

Even though the company has sent 100,000 doses of the vaccine so far to the WHO, it still has on hand about 300,000 doses in vials and in bulk, Coller said. Converting bulk vaccine to vials that are ready to ship takes four or five months, she said.

Merck has been working to replenish the supplies as WHO has drawn down on the stockpile. It has been helped in that by a quirk in timing.

The company started building up the Ebola vaccine stockpile in 2015 and 2016. Recognizing that some of the earliest stock would be coming up to its expiry date, Merck began making fresh supplies about a year ago.

Those earlier doses did not expire—they were used in North Kivu, Coller said. And that move to top up the supply means the stockpile is still holding at about its 300,000-dose set point.

“Based on uncertainty around the outbreak, we’re also exploring options to even expand upon that and to potentially kick in additional manufacturing that was frankly not planned as part of our normal replenishment effort,” Coller said.

Republished with permission from STAT. This article originally appeared on January 22, 2019.